1. Field of the Invention
The present invention relates to a method for forming a phosphor screen used for display units such as cathode ray tubes and plasma display panels (PDP).
2. Description of the Related Art
Conventionally, dot shaped or stripe shaped phosphor layers containing phosphors that emit light of blue, green, and red are formed on the inner surface of a face plate of a color cathode ray tube. In the color cathode ray tube, an electron beam strikes the phosphor layers and thereby the phosphors emit light of blue, green, and red. Thus, the color cathode ray tube displays a picture. In the color cathode ray tube, filter layers corresponding to colors that phosphors emit are disposed on the front surface of the phosphor layers (namely, between the inner surface of the face plate and the phosphor layers). The filter layers are structured by forming pigment layers in a predetermined pattern between the face panel and the phosphor layers. The pigment layers contain pigments corresponding to respective colors and transmit light with almost the same wave lengths of the light of colors of the phosphor layers. Green and blue components of incident light are absorbed by a red pigment layer. Green and red components of incident light are absorbed by a blue pigment layer. Blue and red components of incident light are absorbed by a green pigment layer. Thus, characteristics of such as contrast and color impurity of a picture are improved.
Conventionally, the filter layers are formed by coating pigment layers on the inner surface of the face plate and performing an exposing step and a developing step so as to pattern the pigment layers. At this point, on the inner surface of the face plate, the pigment layers should have adhesion in an area for which they are left as a pattern of the filter layers. In addition, the pigment layers should have peel-off property in an area from which they are removed. Moreover, since the pigment layers should have transparency, the particles of the pigments should be equally dispersed, not cohered. Phosphor layers with colors corresponding to individual pigment layers are formed on the filter layers by slurry method or the like.
However, in such a forming method of the phosphor layers, phosphors with difference colors reside in the filter layers (pigment layers). For example, when a blue phosphor layer is formed by the slurry method, the blue phosphor resides in the green and red filter layers. Thereafter, when a green phosphor layer is formed, green phosphor resides in the red filter layer. Thus, the uniformity property of a color cathode ray tube deteriorates.
Although the reason of which phosphor resides in the filter layers is not clear, the following reason can be supposed. Pigment particles that compose the filter layers are metal oxide. In addition, when the filter layers are formed, a high molecular compound (resin) is added. Thus, static electric force works between silica used for the surface treatment of the phosphor and the filter layers. The static electric force causes the phosphor to reside in the filter layers. Generally, since silica is negatively charged, it is supposed that the filter layers are positively charged.
To form the phosphor layers, slurry method is normally used. As a photoresist, a mixture of ammonium dichromate and a solution of polyvinyl alcohol is used. As an exposure light source, an ultra-high-voltage mercury lamp is used. However, the pigments that compose the filter layers have an optical absorption in a band with a wave length of around 365 nm where the pigments optically link with the photoresist. Thus, when the photoresist is exposed, the sensitivity becomes insufficient. In particular, the exposure sensitivity of the photoresist that contacts the filter layers decreases. Thus, after the phosphor layers are developed, phosphor drop out thereof.
The present invention is made from the above-described point of view. An object of the present invention is to provide a method for forming a phosphor screen, the method preventing part of phosphor layers and/or phosphor contained therein from residing in the filter layers when the phosphor layers are removed from the filter layers.
Another object of the present invention is to provide a method for forming a phosphor screen, the method almost preventing phosphor from dropping out of the filter layers after the phosphor layers are developed.
A further object of the present invention is to provide a method for forming a phosphor screen that contributes to displaying a picture with high brightness and high contrast, the method almost preventing the uniformity property of a color cathode ray tube or the like from deteriorating.